1. Field of the Invention
This invention relates to thermal insulating composites, and more particularly to an improved thermal insulation composite which is principally of metal, is of low conductivity, relatively lightweight, and which may be fabricated into structural forms of significant mechanical strength and chemical stability.
2. Description of the Prior Art
The prior art includes numerous types of insulations, each having particular physical and thermal characteristics, thereby rending these different materials particularly adaptable to a given application.
For example, U.S. Pat. No. 273,688 of Mar. 6, 1883 describes a metal mesh positioned adjacent to a conduit so as to keep a thermally non-conducting plastic mass or cement spaced from the conduit thus forming a dead air space between the conduit and the non-conducting mass.
U.S. Pat. No. 2,179,057 of Nov. 7, 1939 describes the use of an asbestos paper having nibs on it. Nibbed sheets are laid up such that each nibbed section forms an air cell. Also disclosed is the use of aluminum foil laminated to the asbestos paper nibbed sheets.
U.S. Pat. No. 2,514,170 of July 4, 1950 relates to a thermal insulation for jet engines and the like in which various forms are illustrated. In one form, an open metal mesh supports a pocket filled with asbestos and the like in spaced relation to a tubular member. In another form, a radiation shield in the form of a foil is used, the outer surface being an asbestos cloth. Several other forms of thermal insulation are shown, each using wire mesh members.
U.S. Pat. No. 3,007,596 of Nov. 7, 1961 describes an insulation made up of alternate layers of radiation shield material and an insulation material such as glass wool.
For high temperature use, e.g., 200.degree. F. to 4000.degree. F., various materials are commercially available, but are non-metallic in nature. Typical are aluminum silicate powders, silica fibers, zirconium oxide powders, potassium titinate, glass fibers, aluminum fibers, expanded pearlite, collodial silica and silica aerogels. These materials may be used as ceramic foams (silica carbide, aluminum zirconia and silica) with organic binders such as an epoxy resin. Multilayer composites of fiber glass or foam blankets are also available.
While certain of these materials have low thermal conductivity, e.g., about 0.2 btu/hr.-ft.sup.2 -.degree.F./inch for mixture of powders and polymer binders, they also have high densities, for example 20 to 30 lbs/ft.sup.3, although some commercially available materials, in the form of batting, have densities as low as 3.5 lbs/ft.sup.3.
As a general rule, these materials have low compressive strengths, i.e., about 3600 lb/in.sup.2 at densities of 30 lbs/ft.sup.3. Moreover, there are serious limitations on the environments in which they can be used, e.g., chemical, structural, etc. In some cases, there may be erosion problems or moisture affinity or affinity for certain gases which adversely affect performance of the insulation system. Weight and thickness may also present some problems especially when the insulation is to be used in the aircraft or aerospace fields, in that in order to provide adequate protection, the insulation may be too bulky, too heavy or inherently incapable of providing the structural strengths needed for the severe physical environment in which the insulation is to be used.
For example, in helicopter rotor blades which are designed to provide a gas duct for engine exhaust, it is desirable to insulate the interior surface of the rotor blade from the exhaust gases. In such an application, the exhaust gases may be as high as 1000.degree. F. at a pressure of 40 pounds per square inch absolute with an internal flow Mach number of 0.45. From these extreme conditions inside the rotor blade, it is desirable to limit the temperature of the rotor blade to a value such as 180.degree. F. for several reasons. At a temperature of 180.degree. F., the blade can be formed from conventional materials. This would be advantageous at least from economical and structural stand points. Also, by decreasing the temperature of the blade, the infrared signature is lowered so that the helicopter cannot be as easily detected at night. This is desirable in military applications.
In such a severe environment an insulation having physical characteristics for withstanding a temperature such as 1000.degree. F. and thermal characteristics including a thermal conductivity as low as air would be quite desirable. The insulation systems presently available which have these characteristics unfortunately are deficient structurally. For example, some prior art and presently available insulation systems include chalk-like materials which have a tendency to crumble and spall. These characteristics have made such insulation systems not well suited for use in severe environments such as a rotor blade where extreme vibration and centrifugal loads are present. The chalk-like materials are moisture absorbent and chemically reactive with exhaust products. High temperatures may vaporize the moisture and alter the composition of the insulation itself to produce undesirable gases within the insulation. This is typically referred to as outgassing. Furthermore, in proximity to the high velocity fluids, these materials can be expected to erode so as to decrease the thickness of the insulation. A reduced thickness would be detrimental not only to the structural characteristics but also to the thermal characteristics of the insulation.
It is well known that air space is an effective insulation medium at least when conduction is the primary mode of heat transfer. Within an air space, however, modes of heat transfer other than conduction can be of importance. For example, it is known that in a gravity field air will circulate and thereby carry heat from a hotter surface to a colder surface. This is referred to as natural convection.
Heat also can be transferred in an air space if the air is blown or forced between the surfaces. This is commonly known as forced convection. Finally, heat can be transferred through an air space by radiation wherein rays emitted from a hot surface impinge upon and thereby heat a colder surface.
In an attempt to minimize the heat transfer by natural and forced convection, air has been confined by various apparatus to create the air spaces. It will be appreciated, however, that in order to adapt an air space insulation system to severe environments, it is desirable that the confining apparatus have characteristics for withstanding the severe conditions. Many of the confining apparatus of the prior art have had thermal expansion characteristics which have caused them to elongate excessively. In addition, many of the confining apparatus of the prior art have been incapable of withstanding the extreme temperatures such as 1000.degree. F.
To compound the problem, the confining apparatus of the prior art have not included means for venting the air spaces and, as a result, pressure differentials have developed across the skin sheets. Furthermore, some of the confining apparatus have been moisture absorbent. High temperatures have vaporized the moisture and thereby created pressures within the air spaces. These internal pressures have created significant forces on the skin sheets and resulted in damage to the insulations.
Means for attaching the insulation to a surface have been particularly ineffective where vibration has been severe. Also, when the air spaces have been particularly large, the convection of heat has been a detrimental factor. Means for attaching the elements of prior insulation systems to one another or to a supporting structure have severely degraded the thermal properties of the insulation in order to attain the structural integrity desired. Furthermore, the confining apparatus of the prior art have been particularly heavy especially for use in an aircraft.
Typical of the fields of use for thermal insulation are ducting for high temperature fluids at high or low pressures. Fire walls for fuel tanks on aircraft or engine fire walls, and fire walls for ground structures are another typical area of us. In the case of fire walls in aircraft, the Federal Aeronautics Administration currently specifies that the fire wall be capable of withstanding temperatures of 1800.degree. F. for ten minutes. These criteria have been met by the use of solid sheet steel plates having a density of 494 lbs/ft.sup.3, an extremely high density for materials used for such ancillary, but necessary equipment.